sandbox: Use system headers first for sandbox's os.c

This file must be compiled with system headers, even if U-Boot has headers
of the same name. The existing solution for this is good enough for libfdt,
but fails when we have headers like stdint.h in U-Boot.

Use -idirafter instead of -I, and remove the -nostdinc and other things
that we don't want for this file. The best way to do this is to keep a
copy of the original flags, rather than trying to filter them later.

Signed-off-by: Simon Glass <sjg@chromium.org>